We’ve embedded two demo videos after the break. They show [Peter] first using this hardware to receive MIDI signals from a keyboard on his Android phone, and then he demonstrates using the phone and an on-screen musical keyboard to transmit data back to a MIDI device which generates the intended sounds.

It’s an interesting project and he’s headed down the kit-production path right now. You’ll want to browse all of his recent posts, but we especially liked reading his thoughts about simplifying the circuitry. He originally had two separate voltages running in the circuit with a level converter for data signals. After some re-conceptualization he ditched several components and improved the functionality a bit.

like so many other hobby microcontroller project midi kits out there I don’t see any isolation of the midi ports from the rest of the circuit. older midi devices are very sensitive to these types of microcontroller based adapters not having proper isolation and introducing too little/much current on the midi cable.

and latency isn’t much of a problem from the bluetooth side as midi is already fairly slow and adapting bluetooth signals to midi is only going to add latency in the 1-2 ms range where midi is already at ~ 5-9 ms

@wetware The MIDI circuitry is straight from the specs of the MIDI Manufacturers Association (http://www.midi.org/techspecs/electrispec.php), except that a few resistor values and the buffering of the output have been adjusted for operation at 3.3V. In particular, the MIDI In port connects to an optoisolator and has no electrical connection to the rest of the device. Am I missing anything?

As for latency, you have to consider delays due to buffering, protocol issues, and asymmetries between master and slave. (That’s actually the reason why MIDI over Bluetooth didn’t take off when people first tried it a few years ago; prior to Bluetooth 2.1, latency and jitter were terrible.) Bluetooth 2.1 is supposed to add about 7ms, which is consistent of my measurements of an overall latency of 15-20ms (including everything, Bluetooth and MIDI).

@HAD
There are many Bluetooth protocols. So I don’t think you can turn *any* Bluetooth device into a MIDI controller. There is a re-occurring misconception about Bluetooth in the forum as people keep trying to hack a cell-phone ear-piece into a stereo head set -or- try to get keyboards to work with their Bluetooth media players.

@wetware
I always thought MIDI isolation was to prevent current loops or, worst, to prevent electrocuting someone on stage. But this is a self contained device that runs on batteries. What difference would it make if it had good or bad MIDI isolation?

@nettoyeur: Pay no attention to Whatnot. He considers any home-built project utterly worthless if it costs any more than half what a store-bought solution would cost, even if there’s no store-bought solution. Makes me wonder what he thinks of homework assignments in engineering schools.

I, on the other hand, have been looking for something EXACTLY LIKE THIS! When do you intend to start taking orders?